Engineering economics is the discipline used to evaluate million-dollar investments in infrastructure, manufacturing, and mining. Five indicators hold it up: NPV, IRR, Payback, Break-even, and Sensitivity. This guide presents them at entrepreneur scale — without diluting the rigor, removing the friction.
What engineering economics is
Engineering economics applies mathematical and financial principles to compare investment alternatives and make rational decisions about money over time. It's the methodological framework behind any rigorous financial feasibility analysis.
It was born in industrial engineering, where deciding whether to build a plant or buy machinery required serious methodology. Its tools work just as well for a small venture as for a multinational — regardless of scale.
The standard academic reference is Blank & Tarquin, Engineering Economy (McGraw-Hill, 8th edition). The same book taught at the most demanding industrial engineering programs.
Want to skip ahead and run the numbers instead of reading? Load your project into the dashboard — it runs this entire methodology in under five minutes.
The core concept: the time value of money
All of engineering economics starts from one principle: $1,000 today is worth more than $1,000 tomorrow. Three reasons:
- Opportunity. Today, you can invest it and generate returns.
- Inflation. Over time, $1,000 buys less.
- Risk. Future money carries uncertainty. Today's money doesn't.
The discount rate (MARR) captures all three factors. It lets you compare cash flows at different points in time as if they were all expressed in today's dollars. Without that step, comparing year 1 with year 5 is comparing apples to oranges.
The 5 fundamental metrics
1. NPV — Net Present Value
The question: how much is this investment worth in today's dollars?
NPV = −I₀ + Σ [ CFₜ / (1 + r)ᵗ ]
Criterio de decisión
- NPV > 0
- Accept — the project creates value
- NPV ≤ 0
- Reject — doesn't beat the minimum required return
This is the queen metric. It measures the absolute value the investment creates above the minimum required. Complete NPV guide →
2. IRR — Internal Rate of Return
The question: at what interest rate does this investment yield?
0 = −I₀ + Σ [ CFₜ / (1 + IRR)ᵗ ]
Criterio de decisión
- IRR > MARR
- Accept
- IRR < MARR
- Reject
Intuitive and easy to communicate. It has limits: it may not exist, or be multiple, on projects with irregular flows. IRR vs. NPV in detail →
3. Payback — Recovery Period
The question: when do I get my investment back?
Payback = t such that Σ CFₜ ≥ 0
Donde
- t
- Period in which cumulative flow crosses zero
Desarrollo
- 1.Simple payback: sum nominal flows until I₀ is recovered
- 2.Discounted payback: same, but each CFₜ discounted at MARR (more conservative)
The simple version ignores time value of money. The discounted one respects it. Neither measures profitability — they're risk and liquidity metrics, not value-creation. When (and when not) to use Payback →
4. Break-Even Point
The question: how much do I have to sell to stop losing money?
BE = FC / (p − vc) · MS = (V − BE) / V
Donde
- BE
- Break-Even Point[units]
- MS
- Margin of Safety[%]
- V
- Actual sales for the period
Break-even is independent of NPV — it evaluates monthly operations, not the long-term project. The two views are complementary.
5. Sensitivity Analysis
The question: which variables most impact profitability?
ΔNPVₓ = NPV(X · 1.10) − NPV(X · 0.90)
Donde
- X
- Variable analyzed (price, volume, cost, etc.)
- ΔNPVₓ
- Range of NPV impact when X varies by ±10%
Identifies critical risks. The tornado chart sorts variables from highest to lowest sensitivity. Sensitivity analysis in detail →
The five metrics aren't interchangeable. Each answers a different question. A professional analysis uses them all — and interprets them together. Try them at once on your project and see how they complement each other.
The complete workflow
Step 1: Define the project
├── CAPEX (initial investment)
├── Working capital
├── Time horizon
└── Residual value
Step 2: Project cash flows
├── Revenue = Price × Volume × (1 + Growth)^t
├── Variable costs = Unit VC × Volume
├── Fixed costs = Monthly FC × 12
├── Depreciation (tax shield)
└── Tax on EBIT
Step 3: Set MARR
├── Opportunity cost
└── Project risk premium
Step 4: Calculate NPV, IRR, Payback, BE, Sensitivity
Step 5: Interpret and decide
Each step depends on the previous one. An error in MARR (Step 3) propagates to NPV, IRR, and discounted Payback. An error in the flows (Step 2) corrupts them all. That's why the analysis's quality is the quality of the assumptions.
Common mistakes that invalidate the analysis
| Mistake | Consequence |
|---|---|
| Projecting unrealistic growth (50% annually) | Inflated NPV, incorrect decision |
| MARR below inflation | Positive NPV even though the project loses purchasing power |
| Not including working capital | Underestimation of initial investment |
| Mixing nominal rate with real flows | Wrong NPV, may be positive or negative for the wrong reasons |
| Ignoring asset residual value | Underestimation of NPV on infrastructure-heavy projects |
| No sensitivity analysis | Critical project risks unknown |
Run the analysis without building spreadsheets
Factibilidad.io implements the entire workflow in an interface built for entrepreneurs. You enter the parameters and get:
- NPV, IRR, Equivalent Annual Worth (EAW), Profitability Index.
- Simple and discounted Payback.
- Break-Even in units and dollars + Margin of Safety.
- Sensitivity Analysis with tornado chart.
- Three automatic scenarios (base, optimistic, pessimistic).
- Inflation correction using the Fisher equation.
All based on Blank & Tarquin's formulas, with step-by-step verification available in the calculation panel.
Apply what you read — load your project and get the complete analysis.
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